Various methods and systems are known for generating short wavelength radiation. For example, x-rays may be generated by striking a target material with a form of energy such as an electron beam, a proton beam, or a light source such as a laser. Extreme ultraviolet radiation (EUV) may also be generated in a similar manner. Various forms of short-wavelength radiation generating targets are known. These known systems and methods typically irradiate gases, liquids, frozen liquids, or solids to generate the short-wavelength radiation. Current systems that use either room temperature liquid or gas targets impose limitations on the type of chemical elements or materials that can be irradiated because many elements are not in the liquid or gaseous state at ambient pressure and temperature. Hence, the range of desired wavelengths achievable by either gas or liquid systems is also limited.
Solid materials provide a wide range of short-wavelength emissions currently unavailable in materials that are in a liquid or gaseous state at ambient temperature and pressure. One type of prior x-ray generation system uses solid blocks of material (e.g., copper) to generate laser plasma x-rays. In this system, a block of material remains stationary in the irradiation area while laser beam pulses repeatedly irradiate the block of material to produce plasma. The laser beam generates temperatures well over one million degrees Kelvin and pressures well over one million atmospheres on the surface of the material. These extreme temperatures and pressures cause ion ablation and send strong shocks into the solid material. Ion ablation from the surface of the target material at very high speeds and temperatures causes contamination within the radiation chamber as well as to other system equipment such as the radiation collection system and the optics associated with the laser. Thick solid targets induce shock waves that reflect back from the target surface and splash the x-ray chamber with target debris. Ion ablation and target debris decrease the efficiency of the system, increase replacement costs, and shorten the lifetime of the optical and laser equipment.
Another form of solid target material is a very thin tape of target material (e.g., copper (Cu) tape for 1 nm and tin (Sn) tape for 13.5 nm radiation). In these systems, a roll of target tape is dispensed at a predetermined rate while a laser beam pulse irradiates and heats the tape at a desired frequency. The fast ions ablated from the target surface are ejected away from the target. The plasma-generated shock wave breaks through the tape and ejects most of the target material at the back of the target where it can be collected. Thus, use of this tape target reduces ion contamination within the x-ray chamber when compared with solid blocks of target material. Unfortunately, the use of a thin tape target does not completely eliminate target debris at the laser focal point of the target tape. To eliminate or further reduce material contamination within the x-ray chamber, the radiation chamber is typically filled with an inert gas (e.g., helium) at atmospheric pressure. As target ions are ablated from the target material, helium atoms collide with the high-velocity ions, stopping the ions within a few centimeters from the target position. As the helium gas/ion mixture is re-circulated within the radiation chamber, filters trap the ions, recirculating only the helium gas at the completion of the filtration process. The use of thin tape targets and helium gas to stop ablated ions from contaminating the radiation chamber is described in more detail in Turcu, et al., High Power X-ray Point Source For Next Generation Lithography, Proc. SPIE, vol. 3767, pp. 21-32, (1999), incorporated by reference in its entirety into this application. Unfortunately, significant amounts of target debris can still be produced in cooler portions of the laser beam. Moreover, this system does not provide mechanisms that deflect target debris away from optics, and other expensive equipment used in generating radiation.
Current systems and methods utilizing thin tape targets suffer additional disadvantages. The types of materials that are commercially available in thin tape form are extremely limited. Further, thin tape targets require a large tape-dispensing apparatus, which utilizes a significant amount of space within the x-ray chamber, substantially adding to the size and space requirements of such x-ray generators. Tape targets also require frequent reloading of new tape material, which disrupts the operation of the x-ray generator. For example, a reel of thin tape target material having a length of approximately one mile, with a reel diameter of approximately eight inches, typically needs to be replaced with a new reel of tape after a few days of continuous x-ray generation.
The ideal target for a laser-produced plasma should therefore possess the following characteristics. First, the target should be a thin disc with a diameter that matches the focal spot size of the laser beam. The disc should preferably be normal to the laser optical axis. Second, the thickness of the target disc should be minimized to ensure that the laser illuminates all of the target material and therefore formed into plasma. A thin target disc also minimizes ion ablation and shock wave dispersal of the target material. Third, a thin target disc allows more efficient targets to be used. For example, some materials, such as tin or copper, have relatively high conversion efficiencies. Fourth, by utilizing limited amounts of target material in the discs, the amount of debris generated during illumination can be minimized.
In view of this information, a need exists for a method and system that provides short wavelength radiation over a broad range (including x-rays and extreme ultraviolet), with minimum target debris and equipment contamination. There is also a need for short-wavelength radiation-generating targets that approximate a thin disc comprising the target material.